The majority of audible telecommunications are carried on by means of telephone instruments which include a hand set with a microphone and an earphone. The microphone transmits audible voice utterances from a user toward a transmission medium and the earphone produces audible sounds in response to signals from the transmission medium. Hence the microphone is often referred to as a transmitter and the earphone is often referred to as a receiver. The transmitter is coupled with a transmit path via a pair of wires and the receiver is coupled with a receive path via another pair of wires. The transmitter and the receiver are spaced apart in the hand set such that in normal use any acoustical energy coupled from the transmit path to the receive path via the transmitter and the receiver is insignificant. Although the majority of telephone calls are conducted exclusively with hand sets, there are instances where the requirement of having to hold the hand set adjacent ones mouth and ear is sufficiently inconvenient to encourage the usage of a loudspeaking telephone wherein signals are amplified at the receiver to be widely audible and the transmitter is of greater sensitivity to be able to pick up the user's voice at a distance. Consequently, acoustical energy coupled from the transmit path to the receive path via the transmitter and the receiver is sufficient to induce singing, were it not for additional means such as voice switchers or echo cancellers used in the typical loudspeaking telephone. If the loudspeaking telephone is coupled with a bidirectional two wire telephone line, such coupling is by way of a hybrid circuit which being far from perfect, may exacerbate the singing problem. Nevertheless each of these means introduces side effects or fails to perform sufficiently well to make the technology of hands free telephony desirably unobtrusive.
Design efforts to achieve unobtrusive voice switching have spanned many decades. For example, A. E. Bachelet et al in U.S. Pat. No. 2,696,529, titled "Voice Operated Switching System" and issued on Dec. 7, 1954, taught a voice switch, having a relay being controlled in response to signal energies for selectively coupling transmit and receive information signals between a four wire system and a half-duplex transmission medium. Shortly thereafter, L. E. Ryall in U.S. Pat. No. 2,702,319, titled "Two-Way Telecommunication System" issued on Feb. 15, 1955, discussed requirements of a voice switching system as being:
(a) Rapid operation of the switches so as to minimize the initial increment of voice energy lost in switching;
(b) A delay or hangover in restoring to normal, sufficiently long to give continuity of speech between syllables and to suppress reverberation or echo;
(c) Facility for either party to break in during the hangover time;
(d) A hangover time which does not vary unduly with variations in signal strength; and
(e) Protection of each signal path from the effects of signal leakage between them or acoustic coupling between them.
Twenty years of technological advance and design evolution toward optimally meeting these requirements are exemplified by J. L. E. Thompson et al in their U.S. Pat. No. 3,889,059 titled "Loudspeaking Communication Terminal Apparatus And Method Of Operation" issued on June 10, 1975, and assigned to Northern Electric Company Limited. In spite of the use of highly developed analog discreet and integrated solid state circuit components, arranged in circuits, tailored to optimize the performance of these requirements in the speakerphone, the presence of the voice switch continued to be obtrusive. One of the obtrusive characteristics is apparently associated with the rate and an amount of loss which must be switched to alternate between receive and transmit modes of operation. Realizing this, Thompson et al introduced an idle mode of operation, intermediate the transmit or receive modes of operation. The idle mode of operation became effective in the absence of significant speech sound after a short hangover time, whereby a lesser amount of loss is switched to enter the transmit or receive modes. Furthermore the voice switch was restrained from fully entering the transmit mode in accordance with a presence of monotonous sounds which are characteristic of background noise and uncharacteristic of speech. Typically in Thompson et al's method, a transition to either the transmit mode or to the receive mode is frequently achieved with less than the full amount of loss being switched The resultant operation of the voice switch was sufficiently unobtrusive that a hands free telephone of this general design became a moderate commercial success. Succeeding designs have largely replaced the analog circuit elements of the voice switch with digital devices and a digital controller, which together execute the voice switching functional requirements with precisely tailored consistency and reduced obtrusiveness. However in all but the most favorable conditions in a telephone call, one party making use of a typical telephone handset will be aware of an unpleasant feeling of, from time to time, being momentarily cut off if the other party is using a voice switching speakerphone.
The availability of digital circuits at relatively low cost may have encouraged S. Bernard et al to attempt to avoid the inherent obtrusiveness of voice switching by an alternate arrangement which includes transversal filters adapted to perform echo cancelling functions for sounds coupled between the transmit and receive paths. It is known that acoustical energies from the loudspeaker, sufficient to contribute to singing, tend to be dissipated and become insignificant, after traversing distances of twenty feet or so, that is with a time delay in excess of about twenty milliseconds. It is further known that significant electrical analog signal energies may be leaked across a hybrid circuit, usually with a delay of not more than about eight milliseconds. In their U.S. Pat. No. 4,225,754, titled "Loudspeaker Telephone" and issued on Sept. 30, 1980, Bernard et al teach a first transversal filter for synthesizing an acoustic echo signal which is subtracted from the microphone signal with the objective of delivering a substantially echo-free signal to the transmit channel. Likewise, a second transversal filter synthesizes an electric echo signal to deliver a substantially echo-free signal to the reception channel. Practical embodiments have achieved speakerphone operation, without singing, but operation is often obtrusive because of reverberant echoes for which the first transversal filter is not effective.
In U.S. Pat. No. 4,578,543, titled "Digital Echo Canceller" issued on Mar. 25, 1986, Jean Le Bourlot and Michael Levy improve upon this situation by offering an echo canceller provided by a digital non-recursive time domain transversal filter which is effective over twice the time of practical prior filters. However, Le Bourlot et al are merely exemplary of attempts to provide practical transversal filters with extended echo cancellation capabilities. Attempts to provide adequate echo cancellation through typical reverberation times associated with hands free telephony have been plagued with problems such as; poor discrimination of low energy speech signals in the presence of background noise, exaggerations of quantitization distortion, and noticeably slow rates of initial adaptation convergence. In particular some attempts to hasten the rate of convergence have lead to instabilities resulting in divergence, sufficient to cause singing.
It is an object of the invention to provide for hands free communications while substantially avoiding undesirable operating side effects of echo cancellers and of voice switches.